Synthesis of Chiral α,δ-Dioxygenated Allylic Stannanes as Reagents for Carbohydrate Synthesis and Homologation

The δ-oxygenated allylic stannanes 4.4 and 4.5, prepared through addition of Bu3SnLi to γ-OTBS crotonaldehyde 4.3c followed by etherification of the adduct with TBS-Cl or MOM-Cl, undergo transmetalation with InCl3 and in situ addition to aldehydes leading to mainly anti adducts 5.1 or 5.2, accompanied by varying amounts of syn diastereomers. Selectivities of >95:5 can be realized with the MOM reagent 4.5 and ynals 4.3d and 4.3e or cyclohexanecarboxaldehyde 4.3a. With enals 4.3b and 4.3c, 80:20 mixtures of anti and syn adducts are formed. The S enantiomer 10.1 of stannane 4.5 has also been prepared as a reagent for carbohydrate synthesis. Accordingly, addition to α-ODPS acetaldehyde 10.2 in the presence of InCl3 leads to the adduct 10.3 as an inseparable 90:10 mixture of anti and syn diastereomers. Dihydroxylation of the OTBS derivative 10.4 affords the potential altrose precursor 10.5 in 81% yield.